Put on Harley Muffler--now backfires??

[BugeyeNJ58]

Just went for a drive with harley muffler installed. When I accelerate, then rapidly de-accelerate, I'm getting some backfire from the muffler

The muffler now has more back-pressure than previous muffler, could that be a cause? There's also some oil in the muffler, that is slowly burning off as seen by smoke exiting the muffler - car didn't smoke before. The smoke is also lessening as I drive it.

If there is increased back-pressure, would that reduce the rate of blow from the cylinders on the exhaust stroke, would that lean or richen the air/fuel mixture?
Seems like it would lean the mixture.

Thanks....

 

[Jim_Gruber]

How about volume other than backfire and is this the bullet shaped muffler. I tried 2 other types and they sounded like a Rice Rocket Fart Can on Bugsy. Not a good sound.

Also are the baffles in place, there is a screen in there that you can see if you hold up to the light.

Is there an exhaust leak providing backpressure where you clamp to the exhaust pipe?

 

[BugeyeNJ58]

Jim,

It's the bullet shaped muffler like the one in your picture, it sounds good. The baffles are in place, and there is a slight exhaust leak where I clamped the muffler to the existing pipe. I'll be putting J&B weld on it tonight to seal it up.

Good call....hopefully that small leak is causing the noise. Thought it's not a bad noise....sounds kind of cool.

thanks...

 

[Deleted member 8987]

Exhaust leaks before the muffler can cause backfiring on decel.

 

[Pythias]

I just put on a Harley "Shorty". It seems to have a deeper rumble than the old PaceSetter type, but isn't any louder. ..It can't have more backpressure than the old, as you can see straight thru it.

 

[vagt6]

Actually, many Harley bikes have backfires, and popping noises coming from the exhaust system.

Seems normal for most Harleys, and for your car if it has a Harley silencer on it.

 

[Guest]

Punch the center baffle out in the middle of the pipe. That'll reduce your extra back pressure and it still won't be loud. It pushes out towards the back. You might be able to beat it out w/ an old broom stick or piece of pipe. It's just like a freeze plug stuck in the middle of the pipe. The other stuff stays in. I used my 3/4" breaker bar. It'll sound great but not too loud, trust me! Sometimes I can barely here my car running on the interstate. I really should do some vid of it.

 

[BugeyeNJ58]

If you punch out the center baffle, which is still in the muffler, seems that it has to get somewhat louder. I don't want to take that chance as the car is loud enough.

Anybody else punched out the baffle as per Kellysguy?

 

[regularman]

I put a stock sportster muffler on mine. It quietened it a lot and does back fire when I let off in gear down a long hill. This is almost always caused by fresh air getting pulled into the pipe via a leak somewhere from the exhaust manifold on back.
More backpressure will generally limit your power a bit when absolutely wide open throttle but give you a little help on low end torque.
A wide open pipe allows some relatively cool air to go back up the pipe and get pulled into the cylinders.
It has to do with flow like a chimney. With some backpressure, exhaust is still bleeding out the muffler while the next cylinder is firing, keeping a uniform flow.

 

[regularman]

With motorcycles it was always "wide open pipes for wide open throttle. When wide open everything is working as hard as it can and there is always some back pressure there because the exhaust gas can't get out fast enough. At lower throttle positions is where you need back pressure to keep even flow and get better fuel economy because it keeps it flowing in one direction.
Also, opening things up with more flow at the pipe end and at the breather end is not always a good thing. The more air that flows through, the more gas gets mixed with it.
I have seen fuel consumption increase significantly on bikes with straight pipes and more open air cleaners. Now, this did increase power at the top end a bit, but how often are you going to be locked throttle?
I am by no means an expert on this. This is just the things I have learned over the years. The smaller the engine, the greater effect a modification of the exhaust system is going to be.
I have literally had motorcycles sold to me for dirt cheap because the would not run well and people had spent a lot of money trying to get it to run well with expensive new pipes, carbs, intakes, etc.
I have take the same bike an put the stock pipes, intake, carbs, jets back in and it run like an absolute champ.

 

[Jim_Gruber]

You can take a long drill bit and remove the baffle fairly easy. It is not a very substantial screen.

 

[Guest]

More backpressure will generally limit your power a bit when absolutely wide open throttle but give you a little help on low end torque.
A wide open pipe allows some relatively cool air to go back up the pipe and get pulled into the cylinders.

Both of these are old wives' tales.

Gas velocity helps low end torque, not back pressure. A smaller dia longer pipe will give more low end than a larger dia shorter one. This is due to the gas' inertia creating a lower pressure behind each pulse as it travels down the tube helping evacuate the cylinder behind it.

Torque increase with volumetric effiency, not back pressure. Back pressure does not increase torque as it hurts, not helps cylinder filling.

People falsly associate back pressure with this as the smaller pipe creates more of a restriction on top end.

Yes, an eight inch zoomie may introduce some cold air back up the exhaust on a top fuel dragster at idle due to enormous valve overlap. Maybe, and I do say maybe; a short zoomie on a bike MIGHT too. A six foot pipe on a street four cylinder engine won't.

Jim, his center baffle is a plug on the softail type mufflers.

I, as well as everyone else who hears my car at idle, is shocked to learn it's a straight through muffler. It's just as quite as stock.

The 1500 may idle quiter than a 1275. My friend's 1275 w/ sport muffler is louder than mine. Both systems are straight through. If it's too loud for you now, try to bump your timing up just a bit.

I've said this over and over again. The stock Harley center baffles MUST BE PUNCHED OUT to allow proper flow! There were desinged to flow only a little more than HALF of the volume you are now putting through it at any given RPM.


This is way too much back pressure for these engines. Mark the stick before you push it out. That way you can reinstall it if you think it's too loud.

If back pressure induces low end torque, you should be snapping axles at idle.

Kim, the reason you saw fuel consumption increase w/ straight pipes is due to the pulses evacuating the fuel charge through the combustion chamber out the exhaust purging spent gases. The cylinder still gets it's fill, just a little bit is wasted. "Thy cup overfloweth" kinda thing.

Back pressure will reduce/eliminate this, but it doesn't increase power.

 

[BugeyeNJ58]

Those of you who removed the screen, did your car get louder?

Also, why would advancing the timing make the car less loud?
"If it's too loud for you now, try to bump your timing up just a bit."

What will to much back pressure do? Will it cause oil leaks?

 

[Guest]

Those of you who removed the screen, did your car get louder?

It's not a screen, it's a plug and no, it didn't get any louder. ( It should be a plug in yours unless they changed the design.)

Also, why would advancing the timing make the car less loud?

More time b/w bang and exhaust valve opening. Think of it as a gunshot in a room w/ the door closed. The sooner you open the door relative to the shot being fired, the louder it will sound outside the room. Try it. you'll see. No, don't shoot inside you house, advance your timing.

What will to much back pressure do? Will it cause oil leaks?

Well, I imagine if your rings and valve guides are really worn and you have little to no PCV, it might make it leak or make a weak leak worse. That's a stretch though. In effect it would be the combination of worn parts and the excessive presure but it shouldn't cause it on it's own.

Too much pressure will rob you of power and economy, make your engine run hotter and harder. It's not good for it at all.

 

[BugeyeNJ58]

I'll be working on my muffler today. Here's some information about a cheap/easy way to make a tunable baffle on the harley mufflers. Costs about $1.00

https://www.nightrider.com/biketech/exhaust.htm


Thanks for the info.

 

[Guest]

I wouldn't do that mod as IMO it's not needed in your case. Looks like they are using exhaust restriction to make up for improper jetting or using the bolt as an anti reversion device to keep exhaust from being "drawn" back in at lower RPM's.

We have to compare apples to apples. Your car is a 4-1 system, not a 2-2. Drag pipes on a bike use one pipe per cylinder and are much shorter than the one on your car. One one pulse goes down drag pipes v.s four on your longer pipe. The other three pulses plus the extra lenth of you pipe creates a low pressure area in the other three ports exhause ports that aren't being fired. This helps "draw" the air/fuel charge into the combustion chamber durring split overlap ( bloth valves open at the same time on the intake stroke). It also keeps spent exhaust gases from entering the combustion chamber.

This is what is happening with the drag pipes on bikes. One pulse down a shorter tube @ lower RPM is effectivly just a open tube when the next intake stroke comes around. The energy and gases from the last stroke are long gone. As the pistom moves down on the intake stroke, it is allowed to "draw" "air" from both the intake AND exhaust freely. This dilutes the air/fuel charge and produces less power.

Now lets put pressure in the exhaust for the same situation. There will be more pressure on the exhaust side than the intake side and exhast gases quickly move into the combustion chamber at a much greater rate than the intake charge. This would further reduce the amount of air/fuel charge. This is one reason why more back pressure is bad.

Now, let's stick a restriction in the end of the drag pipe. When the piston moves down, it creates a low pressure in the combustion chamber and "draws" more in from the intake than exhaust. The exhaust gases have a harder time traveling backwards into the combustion chamber as atmospheric pressure on the outside of the tube is now restricted. Atmospheric pressure on the intake side is greater, therfore; the air/fuel charge is "drawn" (forced) into the cylinder.

Before this, we had the carb butterfly and air filter restricting the intake, but we had a wide open exhaust tube. Which one you think it's gonna "draw" more in from? You have to remember, both intake AND exhaust valves are open at this time.

An engine is a pressure differential machine. It works by differing pressures on various parts at different times. It's a difficult concept to get you mind around at first. We all have thought in terms of "suck and blow" but not a difference in pressure. I "learned" this at 17, and it took me a at least three years to get my head around it. It took it as fact simply because I trusted Herr Instruktor and was tired of having five pound text books thrown at me when I disagreed. (I love that old man!)

In your case, you have the other three cylinder's exhaust keeping the gases from traveling backwards and atmospheric pressures out.

I have great low end and the baffle is knocked out.


I recant my earlier statement on fuel comsumption w/ drag pipes. After explaining this I can see that's not the case and more likely due to having to richen up the fuel mixture.


If you have access to a welding machine, I can tell you how to take out the whole baffle assembly and drill more holes in it and put it back in.

 

[Guest]

I would like to say that on a two stroke and some forced induction applications, some "back pressure" is required to keep intake charge from being blown out the exhaust port.

Some "back pressure" is bennificial if you have a positive pressure on the intake side.

 

[BugeyeNJ58]

Wow....thanks for that great writeup. I also think in terms of "suck and blow" , not the difference in pressure. Your explanation really helps to understand what is happening internally.

I'll let you know how it works after I take the center baffle out.

 

[BugeyeNJ58]

Well, I punched a hole in the plug, and now the car is much louder, as I thought.
Not sure what to do now. I could try to repair the plug I punctured but then I'm back to the question of excessive "back pressure".

From what I've read that can cause over heating and other issues.

Puzzled in NJ.

 

[Guest]

You shouldn't have punch a hole in it. You should have punched it out. It comes out. Take a 5/8" or so socket and put it on every extension you have. Insert it in the back of the muffler. Take a hammer and beat on the end of the extension sticking out of the back of the mufler. The plug will come out. Either weld up the hole or go match up a new plug at the parts store. Reinstall the plug.


O.K, going to the point of no return. Take a hole saw that fits just inside the outlet of the muffler. It's 1 7/8" or 2" or something like that. You want it to fit just inside the outlet of the muffler. We are going to cut out the welds that hold the baffle in.

Crank up drill and start cutting. Spray some type of cutting fluid, WD40, motor oil, cat urine or something else.

Once through, pull the inner baffler assmeble out towards you. It should come right out. You'll see a series of small holes in the inner tube. Make more small holes like this all the way up and down the tube completely around it. Stick it back in the muffler and weld it up. You should have already put the plug back in, or, you can install it later if this is still too loud. The plug should go in the middle of the holed area. In other words, if the pipe has holes drilled in it for 8" of it's lenth, install the plug in the middle if this area. You should have 4" of holes on either side of the plug. If ten inches of holes....

You can make the holes bigger too, but it will get louder. Mine is super quite.

I think the 1500 may be quiter due to it's longer stroke.